1. Field of the Invention
The present invention relates to a semiconductor light emitting device and the method of making it, especially to the semiconductor light emitting device that comprises a strong ohmic layer formed between a top semiconductor material layer and a transparent conductive oxide layer, based on which the semiconductor light emitting device has lower and steadier forward voltage.
2. Description of the Prior Art
The transparent conductive oxides, e.g. ITO, ZnO, InO, ZrO, and so on, have been used for overlaying the semiconductor light emitting device. The main function of the transparent conductive oxide layer (TCOL) overlaying the semiconductor light emitting device is to be a conductive medium layer for current diffusion and light passing to improve the external quantum efficiency of the semiconductor light emitting device.
However, the ohm contact between the transparent conductive oxide layer and the top semiconductor material layer (e.g. P-type GaN layer) is not easy to be achieved, The key point of the technology of how the transparent conductive oxide layer is overlaid depends on how the lower and steadier forward voltage (Vf) is achieved. Several prior arts will be described in the following to show the difficulty in the technology.
In U.S. Pat. No. 5,977,566, Okazaki and others propose adding a metal agent layer between an ITO layer and a P-type GaN contact layer to reduce the offset of the conductive area. The material being used to form the above agent layer comprises Mg, Ni, Au, Zu, and Ti. Accordingly, the forward voltage (Vf) of the semiconductor light emitting device is reduced. Moreover, in U.S. Pat. No. 6,078,064, Ming-Jiunn and others propose using a P-type contact layer with highly doped consistency (more than 5×1018) to be the agent layer. In U.S. Pat. No. 6,479,836, Suzuki and others propose selectively doping P-type carrier with high consistency to form super-lattice (e.g. InGaN/GaN, AlGaN/GaN, or other combinations), so as to obtain lower forward voltage (Vf). Furthermore, somebody also proposes using Ni to be the agent layer and utilizing the difused oxygen in the ITO layer to convert it to a NiO layer, so as to reduce the forward voltage (Vf) of the semiconductor light emitting device.
However, in the above arts utilizing an agent layer, the agent layer itself can absorb the intensity of the light outputted by the semiconductor light emitting device, or else during the operation, the carrier is diffused between the agent layer and the contact layer due to the agent layer having highly doped consistency, so that the forward voltage (Vf) of the semiconductor light emitting device may be very unsteady.
The process, which utilizes the ITO layer as the current diffusion layer to improve the intensity of the light outputted by the semiconductor light emitting device and to further overlay the general Ni/Au transparent conductive layers (TCLs), is well-known in the prior arts. For example, in U.S. Pat. No. 5,925,897, Oberman and others propose forming a very thin Au/Ni composite layer between the ITO layer and the P-type InGaN contact layer. Moreover, in U.S. Pat. No. 6,465,808, Lin and others propose a lattice transparent conductive layer. Because the lattice transparent conductive layer has fewer absorption areas, more light emitted by the semiconductor light emitting device can be outputted. In U.S. Pat. No. 6,287,947, Ludowise proposes forming a plurality of transparent conductive layers between the ITO layer and the P-type GaN contact layer. However, the problem of the above technology is that the reproduction of the forward voltage (Vf) and the luminance (Iv) of the semiconductor light emitting device is worse due to the difference of the surface roughness of the epitaxial wafer or the hydrogen passivation effect.
Although there are many different types of ohm contact layers for performing the function of contacting the ITO layer or the transparent conductive oxide layer, the above ohm contact layers are all N-type or P-type with highly doped consistency. During the process of manufacturing the semiconductor light emitting device, the environment of the process, such as residual oxygen or hydrogen, will easily affect the effect of the ohm contact layer. The forward voltage (Vf) of the semiconductor light emitting device will also experience offset caused by unsteady quality of the ohm contact layer.
Therefore, the objective of the present invention is to provide a strong ohmic layer for a semiconductor light emitting device, so that the semiconductor light emitting device can have a lower and steadier forward voltage.